In recent years, there has been considerable effort aimed at understanding new phenomena in the nanoscale, a diversity of new nanostructured materials have been fabricated and characterized. New devices with intriguing properties are just beginning to be engineered. The expectations for a new generation of cheap and innovative tools that will change our lives are very high. The combination of a new set of expected and unexpected properties together with a whole new family of materials and fabrication methods will enable devices that we could not even have conceived just ten years ago. Coulomb blockade in metal nanoparticles as well as in semiconductor quantum dots, narrow band fluorescence emission from semiconductor nanoparticles, quantized ballistic conduction in nanowires and nanotubes: these are just a few new materials/phenomena that will have an impact on the way we design optical and electronics devices. For a review of nanodevices and fabrication techniques, see Bashir, Superlattice and Microstructures (2001), 29(1):1-16; Xia, et al., Chem. Rev. (1999), 99:1823-1848; and Gonsalves, et al., Advanced Materials (2001), 13(10):703-714, the entire teachings of which are incorporated herein by reference.
Nanoscience development, similarly to many other branches of science, but probably in a more extreme way, relies on state-of the-art techniques for the imaging and fabrication of its tools. Undoubtedly, the development of the transmission electron microscope (TEM) and scanning tunneling microscope (STM) gave birth to the whole field of nanoscience. While the development of electron-beam (e-beam) lithography, being the first tool that could build structure and devices in the nanometer scale, made the field of nanotechnology a reality.
The first stages of nanoscience and mainly of nanotechnology have been dominated by the development and characterization of new materials and devices based on inorganic semiconductors and metals. One of the main reasons for this is that e-beam lithography is a technique to pattern inorganic materials on an inorganic substrate. A significant advancement in recent years has been the development of novel highly versatile nanolithographies based on scanning probe microscopes (SPM). Using various types of SPMs a wide variety of organic and inorganic substrates can now be patterned either by inducing localized chemical modifications or by forming self-assembled monolayers (SAMs). For example, Mirkin and coworkers have developed an atomic force microscope (AFM)-based technique (Dip Pen Nanolithography, DPN) in which a SAM can be generated by controlled transfer of molecules from the microscope tip to a substrate, with resolution below 5 nm (see Lee, et al., Science (2002), 295:1702-1705; Demers, et al., Angew. Chem. Int. Ed. (2001), 40(16):3069-3071; Hong, et al., Science (1999), 286:523-525; Piner, et al., Science (1999), 283:661-663; Demers, et al., Angew. Chem. Int. Ed. (2001), 40(16):3071-3073; Demers, et al., Science (2002), 296:1836-1838, U.S. Patent Application Publication Nos. 2002/0063212, 2003/0049381, 2003/0068446, and 2003/0157254, the entire teachings of which are incorporated herein by reference). The development of such techniques represents a major breakthrough, as now it is possible to build devices based not only on inorganic but also on organic and biomaterials. Organic based nanomaterials are likely to offer a number of interesting properties that can be effectively modulated on the nanoscale. Furthermore, the typical disadvantages of organic materials are less important in nanodevices; for example, there is less need for good mechanical properties or high thermal stability. Thanks both to these novel fabrication techniques and to the elucidation of basic concepts in surface and supra-molecular chemistry, novel devices are currently well under development.
Using organic and inorganic based nano-lithography techniques many different nano-devices (e.g nano-transistors, nano-sensors and nano-waveguides) are presently being fabricated. However, in order to predict how great an impact nanotechnology will have, one must estimate the speed of fabrication for complex devices. Unfortunately, all nano-lithographies have in common the same drawback: they are extremely slow, and it has been postulated that device fabrication time (and reproducibility) will be the main limiting factor in nanotechnology. In particular, the problem of how to scale up production has not been solved. Addressing the problem of production scale-up is critical if we hope to see the enormous amount of knowledge that we are now acquiring translated into transistors, sensors, antennas, lenses and drug delivery systems to use in everyday life.
It would be desirable for nanotechnology to have an equivalent of micro-contact printing: this stamping technique engineered by Whitesides and coworkers (see U.S. Pat. Nos. 5,512,131, 5,900,160, 6,048,623, 6,180,239, 6,322,979, 6,518,168, the entire teachings of which are incorporated herein by reference) has revolutionized the way people design micro-devices and has had an enormous impact in allowing non-chemist to build devices as complex as bio-MEMS. Unfortunately, micro-contact printing has serious resolution limitations, so its application in nanotechnology is limited.
The only research efforts to directly address this problem is that by Chou and coworkers at Princeton. In a recent Hewlett Packard press release (“breakthrough in nano-electronics”), their patents and patent applications on nano-imprinting (U.S. Pat. Nos. 5,772,905 and 6,309,580, and U.S. Patent Application Publication Nos. 2002/0167117, 2003/0034329, 2003/0080471, and 2003/0080472, the entire teachings of which are incorporated herein by reference) were considered one of the fundamental steps towards the realization of nano-transistors. The method is based on a hard mold (i.e., a mold made of an inorganic material) that is stamped on a soft polymer film overcoating a silicon wafer. The printed substrates typically consist of metallic wires or semiconductor materials (see Chou, et al., Nature (2002), 417:835-837; and Austin, et al., J. Vac. Sci. Technol. B (2002), 20(2):665-667, the entire teachings of which are incorporated herein by reference). As with many other fundamental aspects of nanotechnology, the fabrication methods for inorganic materials are preceding those for organic materials. In fact, the main limitations to nano-imprint is that it needs a “hard” mold and that it is tailor-made to print a shape on a silicon wafer. It is difficult to envision how such a method could be adapted for soft molds (i.e., a mold made of an organic material) and/or how it could be used to transfer the high degree of complexity and information that an organic (particularly a bio-organic) substrate can carry.
A major drawback of existing nanolithography techniques for fabricating nanoscale devices is that features of the device must be fabricated in a series of steps. Thus, these techniques are limited to relatively simple devices since the fabrication of devices having many features would take a prohibitive amount of time. The only major effort to address this problem is the fabrication of multi-tip arrays for SPMs (Zhang, et al., Nanotechnology (2002), 13:212, the entire teachings of which are incorporated herein by reference). While such approaches will mainly enable the parallel fabrication of a perhaps tens or hundreds of nano-devices, it would be desirable to develop a nanoscale stamping technique that could complement such parallel device production, and move it toward mass-production by developing a method that can produce many features in a parallel manner on a device in a single processing step.